Alkenes => dienes

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Complexes in Which the Acceptor Is a Metal Ion and the Donor is an Alkene or an Aromatic Ring. Note that n donors do not give EDA complexes with metal ions but form covalent bonds instead. Many metal ions form complexes, which are often stable solids, with alkenes, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture... 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Some years ago we began a program to explore the scope of the palladium-catalyzed annulation of alkenes, dienes and alkynes by functionally-substituted aryl and vinylic halides or triflates as a convenient approach to a wide variety of heterocycles and carbocycles. We subsequently reported annulations involving 1,2-, 1,3- and 1,4-dienes unsaturated cyclopropanes and cyclobutanes cyclic and bicyclic alkenes and alkynes, much of which was reviewed in 1999 (Scheme l).1 In recent days our work has concentrated on the annulation of alkynes. Recent developments in this area will be reviewed and some novel palladium migration processes that have been discovered during the course of this work will be discussed. 

Table 1.2 Examples of double and triple bond hydrogenation with recyclable metal NPs in ILs alkenes, dienes, arenes, ketones, and aldehydes. 

The chromatograms of the liquid phase show the presence of smaller and larger hydrocarbons than the parent one. Nevertheless, the main products are n-alkanes and 1-alkenes with a carbon number between 3 to 9 and an equimolar distribution is obtained. The product distribution can be explained by the F-S-S mechanism. Between the peaks of these hydrocarbons, it is possible to observe numerous smaller peaks. They have been identified by mass spectrometry as X-alkenes, dienes and also cyclic compounds (saturated, partially saturated and aromatic). These secondary products start to appear at 400 °C. Of course, their quantities increase at 425 °C. As these hydrocarbons are not seen for the lower temperature, it is possible to imagine that they are secondary reaction products. The analysis of the gaseous phase shows the presence of hydrogen, light alkanes and 1-alkenes. 

Alkynes, alkenes, dienes, allenes, isonitriles, carbonyl compounds, etc. 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Ruthenium complexes are active hydrogenation catalysts for the reduction of dienes to monoenes. Both zerovalent and divalent ruthenium complexes containing various (alkene, diene and phosphine) ligands have been employed as catalysts that have met with different degrees of success. 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

Hunsdiecker reaction, 9, 5 19, 4 Hydration of alkenes, dienes, and alkynes, 13, 1... 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

This methodology was later successfully applied to cyclic alkenes, dienes and trienes [59]. The tentative mechanism proposed by Li et al. involves the activation of the —H bond by gold(I) species (from in situ reduction of gold(III)). Then, the reaction is followed by the alkene attack on the alkylgold hydride intermediate. 

After compiling many results obtained in similar studies of different substrates (alkenes, dienes, alkynes and so on), the results cannot be correlated to draw definitive conclusions due to the wide variety of parameters that can influence the reaction (substrates, catalyst precursors, supports, pressure, temperature and so on) [9, 208-214]. This is maybe the main reason why there are no clear mechanistic explanations for this simple reaction, unlike homogeneous gold-catalyzed processes. 

The latter transformation requires the use of a small amount of an acid or its ammonium salt. By using [Cp2TiMe2] as the catalyst, primary anilines as well as steri-cally hindered tert-alkyl- and sec-alkylamines can be reacted.596 Hydroamination with sterically less hindered amines are very slow. This was explained by a mechanism in which equlibrium between the catalytically active [L1L2Ti=NR] imido complex and ist dimer for sterically hindered amines favors a fast reaction. Lantha-nade metallocenes catalyze the regiospecific addition of primary amines to alkenes, dienes, and alkynes.598 The rates, however, are several orders of magnitude lower than those of the corresponding intramolecular additions. 

Unsaturated hydrocarbons (alkenes, dienes) react with carbon monoxide and a proton source (H20, alcohols, amines, acids) under strong acidic conditions to form carboxylic acids or carboxylic acid derivatives. Since a carbocationic mechanism is operative, not only alkenes but also other compounds that can serve as the carbocation source (alcohols, saturated hydrocarbons) can be carboxylated. Metal catalysts can also effect the carboxylation of alkenes, dienes, alkynes, and alcohols. 

The hydrogenation of many different alkenes, dienes, polyenes, and alkynes may be catalyzed by homogeneous complex catalysts. Many of the soluble complexes have the ability to reduce one particular unsaturated group in the presence of other reducible groups. The selectivity rather than their universality makes these catalysts particularly useful in synthetic organic chemistry. With careful choice of catalyst and reaction conditions, remarkable selectivities are attainable. Most of the practically useful catalysts work under ambient conditions. 

Nickel affords selective catalysts for the hydrogenation of alkenes, dienes, and alkynes. When catalyzed by C. A. Brown s P-2 nickel, prepared by the reduction of Ni(0Ac)2 with NaBH in ethanol, the individual rates as well as the competitive rates appear to be sensitive to the alkene structure as judged by the reported initial rates of hydrogen addition (ref. 23). Alkene isomerization is relatively slow. Except for the most reactive alkenes such as norbornene, the individual hydrogenations seem to be first order in alkene. This indicates that alkenes are more weakly bound to Ni than to Pt or Pd. Similar selectivities are reported by Brunet, Gallois, and Caubere for a catalyst prepared by the reduction of Ni(0Ac)2 with NaH and t-amyl alcohol in THF (ref. 27). 

The complexes [PdCl2(Me2SO)2] and [RhCl3(Me2SO)3] can be reduced by NaBH4 to give catalysts for the isomerization of alkenes and the hydrogenation of alkenes, dienes and alkynes.101... 

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